Resin manufacturers and coating formulators are reducing the volatile organic content (VOC) of their products for safety, health and environmental reasons. One way to reduce VOC is to reduce or eliminate the solvent used to thin the coating composition to a useable viscosity. To do this, the molecular weight of the film-forming polymer binder used in the coating composition must be reduced to compensate for the attendant viscosity increase experienced when the solvent level is reduced. A coating composition with a lower molecular weight binder requires less solvent to thin it to the appropriate application viscosity and, thus, can be formulated at higher solids and lower VOC.
Coating compositions and their constituent binders, which typically have solids contents of about 40% to about 100% by weight, have been labelled "high solids." The low molecular weight polymers in such high solids coatings are typically designed to undergo a crosslinking reaction after the coating is applied to the substrate in order to increase the molecular weight of the polymeric coating and to produce a hard, durable protective finish.
Examples of these non-aqueous coatings include, but are not limited to, alkyd, polyester, isocyanate, polyurethane, epoxy, melamine, urea and acrylic solution polymers and like resins.
For example, to achieve higher solids levels and lower VOC at the same application viscosity of traditional alkyds, alkyd resin manufacturers typically lower the molecular weight of the alkyd resins. Thus before they become dry and tack free, high solids alkyds must undergo more oxidative crosslinking than conventional alkyds.
The long drying time required for high solids coating compositions is a major shortcoming. Various modifiers for high solids coatings aimed at reducing drying time are commercially available. Most of the current modifiers are high glass transition temperature (T.sub.g), low molecular weight solution polymers used to replace a portion of the film-forming polymer in the coating. The high T.sub.g of the modifier speeds up the drying time of the coating, and its low molecular weight does not change the viscosity of the coating composition at a given VOC. Modification with these high T.sub.g, low molecular weight modifiers, however, typically degrades the mechanical properties of the final coating, such as for example by reducing impact resistance and flexibility, and degrades the resistance of the final coating to organic solvents.
Another method for reducing drying time is to add a higher level of metal dryer salt to the coating composition, such as for example, cobalt, magnesium and manganese naphthenate and octoate. Higher levels of these salts, however, degrade and discolor the coating over time.
Thus, there is a need for a modifier to accelerate the drying time of the coating:
(1) without increasing the viscosity of the liquid coating composition;
(2) without decreasing the pot life of the liquid coating composition;
(3) without degrading the properties of the final dry coating;
(4) minimizing the loss of flexibility of the coating caused by the incorporating of particles; and
(5) without discoloring of the final dry coating.